1. Technical Field
The present invention relates to multiple ratio geared transmissions for use in an automotive vehicle powertrain and to a control strategy for effecting engagement and release of transmission friction torque establishing elements during a ratio change.
2. Background Art
A multiple-ratio (i.e., step-ratio) automatic transmission in an automotive vehicle powertrain adjusts a gear ratio between a torque source and a driveshaft to meet drive-ability requirements under dynamically-changing driving conditions. Ratio changes are achieved by engaging a so-called on-coming clutch (“OCC”) as a so-called off-going clutch (“OGC”) is released. The clutches, which may be referred to as transmission friction elements or brakes, establish and disestablish power flow paths from an internal combustion engine to vehicle traction wheels. During acceleration of the vehicle, the overall speed ratio, which is the ratio of transmission input shaft speed to transmission output shaft speed, is reduced as vehicle speed increases for a given engine throttle setting. This is an up-shift.
In the case of a synchronous up-shift, the OCC engages to lower both the gear ratio (i.e., the overall speed ratio) and the torque ratio (the ratio of output torque to input torque). The synchronous up-shift event can be divided into three phases, which may be referred to as a preparatory phase, a torque phase, and an inertia phase. The torque phase is a time period when the OGC torque is controlled to decrease toward a non-significant level with an intention to disengage it. Simultaneously, during the torque phase, the OCC is controlled to increase from a non-significant level, thereby initiating the OCC engagement according to a conventional up-shift control. The clutch engagement and disengagement timing results in a momentary activation of two torque flow paths through the gearing, thereby causing torque delivery to drop momentarily at the transmission output shaft. This condition, which can be referred to as a “torque hole,” occurs before the OGC disengages. A vehicle occupant can perceive a large torque hole as an unpleasant shift shock. The preparatory phase is a time period prior to the torque phase. The inertia phase is a time period when the OGC starts to slip due to substantially reduced holding capacity, following the torque phase.
The release timing of the OGC has to be synchronized with a certain OCC torque level at the end of the torque phase. Missed synchronization leads to inconsistent shift quality, often resulting in audible engine flair or gear-set tie-up with a deeper and wider torque hole.
Certain controls employ an open-loop approach for OCC engagement and OGC release control. This open-loop approach requires manual adjustment of OCC and OGC control parameters under multiple operating conditions. As a result, a manufacturer building a vehicle having the transmission has to carry out a relatively long shift quality calibration process for each vehicle program. It is also difficult to account for variability in actuator characteristics and dynamically changing operating conditions, resulting in inconsistent shift quality.
Other controls employ a closed-loop method to consistently release the OGC at an ideal timing based on direct or indirect measurements of OGC torque. However, this closed-loop method does not provide a solution to mitigate a torque hole.
Other control techniques employ a coupled engine-transmission control during the torque phase to reduce or eliminate torque holes. However, in practice, it is difficult to simultaneously synchronize the behaviors of three actuators (i.e., the engine, the OCC, and the OGC) due to their finite controllability in conjunction with the presence of various noise factors. In order to improve the control robustness, certain control techniques aim at reducing errors between target OCC and OGC torques as compared with those derived from torque sensor measurements within a transmission system. However, engine and transmission controls still remain tightly coupled through kinematic constraints. Synchronization or coupling between engine torque control, OCC engagement control, and OGC release control is still required.
In view of the foregoing, there is a need to reduce the complexity of an up-shift control for improved shift consistency and control robustness.